Interpretive Summary: Leaf-cutter ants are an important pest of tropical and sub-tropical agriculture. They forage over 250 m in habitats that occlude celestial cues for orientation. We investigated their ability to use the Earth’s magnetic field as a directional reference when foraging at night and under vegetation. We moved ants from a foraging trail to a table in a chamber within which we had experimentally reversed the polarity of the local magnetic field. Control ants with the geomagnetic field unaltered oriented directly towards the nest. One-half of the Experimental ants oriented according to the reversed field (geographically 180° opposite to the nest’s direction), indicating that they used a magnetic compass. The other half walked towards the nest suggesting that other cues may have allowed them to orient home. By applying a brief, but strong magnetic pulse, we attempted to disrupt the ants’ magnetic compasses. Control ants that were exposed to a weak biasing field but not a strong pulse oriented homeward. Experimental ants oriented randomly. Thus disrupting the local magnetic field and the ants internal compass resulted in a change in orientation consistent with the use of a magnetic compass to path integrate homeward. Knowledge of how insects orient can be used to improve our ability to predict insect movement between agricultural patches.

Technical Abstract:
Leaf-cutter ants Atta colombica forage over 250 m in structurally-complex, Neotropical rainforests that occlude sun or polarized light cues. Night foraging makes the use of celestial cues and landmarks all the more difficult. We investigated the directional cues used by leaf-cutter ants to orient homeward by experimentally reversing the polarity of the local magnetic field and by experimentally subjecting the ants to a strong magnetic pulse to disrupt a magnetic compass. In both experiments, we transferred homeward-bound ants from a foraging trail to a table in a chamber that occluded landmark and celestial cues. In both experiments, Control ants demonstrated path-integration and walked directly towards the nest. In the reversed field, one-half of the Experimental ants oriented according to the reversed field (geographically 180° opposite to the nest’s direction), indicating that they used a magnetic compass to update their positional reference derived from path-integration. The other half walked towards the nest suggesting that they may have used an egocentric reference to measure their rotation when displaced, although other explanations have not been entirely excluded. With application of a very brief, but strong, magnetic pulse, Experimental ants oriented randomly. We conclude that the leaf-cutter ants use the Earth’s magnetic field as a reference by which to orient when path integrating towards home.